Issues when closing flows

[fredhau]

I experience two issues when flows are closed:

1. If the remote peer closes a connection, on_close() will be called because recv returns 0, but the socket associated with the flow is never closed.
2. If the event loop is stopped via e.g. a signal handler, and neat_free_ctx() is called afterwards, all open sockets are closed, but on_close() is not called for the respective flows. This results in userData associated with the flows not being freed.

I am pretty sure issue 1 is a bug, but I am not sure if issue 2 is a bug.